Actually, they are.
Those rock trucks that actually cause these sorts of things aren't liable,
I think there is some sort of agreement that if a rock carrying truck uses a cover, it protects them from liability. At least that's what I've heard.
In any event, she would have to prove it was your car that threw the rock. Not particularly easy. But who knows, anything can happen in small claims court; judges are usually far from "impartial". One thing is for sure, you don't have to pay her anything unless she takes you to court and actually wins the case. If her insurance doesn't cover her windshield, she is of course more likely to want to go to small claims.
One time here in Arizona, I got a windshield crack from a rock thrown up by a lawn edger. What was I supposed to do, sue the (most likely illegal migrant) lawn maintenance guy? Hell, my stepdaughter's car was totalled by an unemployed, uninsured guy driving on a suspended license. I could have sued him, but gained nothing. He spent a little time in jail anyway ...
Interestingly, that incident led me to buy my Celica. In shopping for a car for my stepdaughter, I found out the Celica was going to be discontinued. I had already endured the Prelude being discontinued without getting one, and decided I really wanted to own a Celica. So I ended up getting two cars in the same deal from the Tempe, AZ, Toyota dealer; my stepdaughter's used Subaru Legacy, and the new Celi.